High temperature use of a self lubricating coating material in a foil bearing and part coated with such a material

ABSTRACT

High temperature use of a self lubricating coating material in a foil bearing and part coated with such a material. A material according to the disclosure includes between 50 and 90 wt % of alumina (Al2O3), as well as an oxide selected from the group consisting of titanium oxide (TiO2), chromium oxide (Cr2O3), and a mixture of these. Application to a rotor and/or a thrust disc at least partially covered with such a material.

FIELD OF THE INVENTION

The present invention relates to a self-lubricating coating material forhigh-temperature use and to a part coated with such a material.

BACKGROUND

Most turbomachinery reaches very high rotational speeds, for examplefrom 20,000 to 120,000 revolutions per minute (rpm). To guide a rotorrotating at such speeds, it is conventional to use foil bearings insteadof conventional ball bearings which are not compatible with suchrotational speeds. A film of ambient air is used to support a rotorwhich is thus “levitated” relative to a fixed housing. Such a bearing istherefore also called a foil-air bearing.

When the rotor rotates at high speed, there is no contact between thevarious solid components of the bearing. However, before and after aphase of high-speed rotation, at startup or shutdown, the film of airwhich serves as a bearing has not been formed in the stationary housing,and a foil, called the top foil, of the foil bearing is in local contactwith both the rotor and the fixed housing that receives them. Means tolimit friction between these elements should then be provided at thecontact surfaces.

It is known to use PTFE (Polytetrafluoroethylene) as a lubricant toprevent wear of the parts in a foil bearing during the startup andshutdown phases. This material is usually covering a foil of the bearingwhich faces the rotating part, also called the top foil, and has goodperformance at relatively low temperatures but cannot be used at hightemperatures (above 200° C. or so).

When the rotor is part of a machine with a relatively powerful electricmotor, the motor is a source of heat. The ambient air used forventilation in this machine is then hot air. Due to the heat of themotor plus the heat locally generated by friction, insufficient heatdissipation may occur and high temperatures of around 300 to 400° C. canbe reached.

U.S. Pat. No. 5,866,518 proposes a composite self-lubricating materialto reduce friction and wear which can be used over a very widetemperature range, from cryogenic temperatures to temperatures up to900° C. This material comprises 60 to 80 wt % (percentage by weight) ofchromium oxide dispersed in a metal binder of an alloy containingchromium and possibly nickel as well as 5 to 20 wt % of a fluoride fromgroups I or II, or a rare earth metal and possibly a metal lubricant.

U.S. Pat. No. 7,297,367 relates to a method for applying an inorganiclubricant coating deposited on a top foil of a foil bearing in order towithstand higher temperatures than inorganic coatings of the prior art.The coating is applied by spraying or by immersion.

The present invention therefore aims to provide a new coating to protectthe foil(s) of a foil bearing from wear and to withstand hightemperatures.

SUMMARY

The invention relates to protecting the usually uncoated top foil of thebearing from wear. It also preferably relates to providing protectionfor thrust discs which may also be subjected to friction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To this end, the invention proposes the use of a coating comprisingbetween 50 and 90 wt % of alumina (Al2O3) and an oxide selected from thegroup consisting of titanium oxide (TiO2), chromium oxide (Cr2O3), and amixture of these, as a self-lubricating coating for the high-temperatureuse of a rotor in a foil bearing.

Tests have shown that such a coating, preferably purely ceramic andwithout metal binder, has good lubricating properties and isparticularly well-suited for withstanding high temperatures. A preferredembodiment provides that the coating used consists of a mixture of 50and 90 wt % of alumina (Al2O3) with an oxide selected from the groupconsisting of titanium oxide (TiO2), chromium oxide (Cr2O3), and amixture of these.

In a preferred embodiment, the coating of the invention is a mixture ofalumina (Al2O3) and titanium oxide (TiO2). Tests have shown thattitanium oxide is preferred to chromium oxide which, however, alsoachieves good performance in terms of lubrication and resistance at hightemperatures. According to these tests, the coating preferably comprisesbetween 65 and 75 wt % of alumina (Al2O3).

The invention also relates to a turbomachine part, comprising a circularcylindrical bearing which is at least partially coated with aself-lubricating coating as described above, in other words a coatingcomprising between 50 and 90 wt % of alumina (Al2O3) as well as an oxideselected from the group consisting of titanium oxide (TiO2), chromiumoxide (Cr2O3), and a mixture of these.

Such a turbomachine part can be such that its self-lubricating coatingis deposited by a thermal spraying process followed by grinding. Thethermal spraying process is preferably an air plasma spraying process.

A turbomachine part according to the invention is, for example, selectedfrom the group of parts comprising rotors and thrust discs.

The invention further relates to a turbomachine, characterized in thatit comprises a turbomachine part as described above. Such a turbomachinethen preferably comprises a foil-air bearing having a housing whichaccommodates, on the one hand, a circular cylindrical portion of aturbomachine part, said circular cylindrical portion being at leastpartially coated with a self-lubricating coating comprising between 50and 90 wt % of alumina (Al2O3) and an oxide selected from the groupconsisting of titanium oxide (TiO2), chromium oxide (Cr2O3), and amixture of these, and on the other hand at least one foil, called thetop foil, coming to face said circular cylindrical portion, said topfoil being untreated.

Finally, the invention relates to an air conditioning device,characterized in that it comprises a turbomachine as described in thepreceding paragraph.

Details and advantages of the invention will become clear from thefollowing description which presents a particular embodiment of theinvention.

The invention relates more particularly to the introduction of aself-lubricating coating onto a rotor or a thrust disc of a turbomachinein which the parts can rotate at high speeds (for example greater than60,000 rpm). Such a turbomachine may or may not be powered electrically.Parts rotating at such high speeds, or the rotor, are usually mounted onfoil bearings and the invention relates more particularly to machinescomprising at least one bearing of this type.

A foil bearing, or foil-air bearing, comprises a housing whichaccommodates a circular cylindrical portion of a rotor. At very highspeeds, a film of air forms between the housing and the cylindricalportion, which prevents contact of the cylindrical portion with theinner wall of the corresponding housing. However, contact may occur whena steady state is not yet established. It is therefore arranged to havea sheet of lubricant material to prevent wear of the housing and/or ofthe corresponding cylindrical portion. This sheet provides lubricationof the bearing in the startup and shutdown phases of the rotating part.

Documents U.S. Pat. No. 7,297,367 and WO 2007/004770 show two differenttypes of foil bearings to which the present invention can be applied.Other types of bearings allowing (very) high rotational speeds can alsobe concerned by the present invention.

A thrust disc is generally associated with a bearing and is intended toprevent longitudinal movement of the rotating part. Here too, it isnecessary to provide lubrication to prevent premature wear of the thrustdiscs during the transitional phases (turbomachine startup andshutdown).

The invention proposes coating a rotor and/or a thrust disc, or at leastthe circular cylindrical portion of a rotor and/or a face of a thrustdisc accommodated by a bearing, with a self-lubricating coating thatallows withstanding high temperatures (for example up to 400° C.).

The proposed coating, in a preferred embodiment, is composed of alumina(Al2O3) and titanium oxide (TiO2) and is applied by a thermal sprayingprocess. This coating then has the aim of protecting from wear thecoated parts in a foil-air bearing and the parts coming into contactwith these coated parts, in an environment where the temperature canvary from −50° C. to +400° C., particularly during transition phases(startup, shutdown, and possibly changes in rotation speed).

While it is usual to coat a top foil of a foil-air bearing withlubricating material, the invention proposes keeping this top foiluntreated, the top foil being the foil of the bearing that comes to facethe circular cylindrical portion of the rotating body. The top foiltherefore has no coating.

First, the invention concerns the coating itself. As indicated, in apreferred embodiment the coating comprises alumina and titanium oxide.The proportion of alumina (Al2O3) is between 50 and 90 wt % of thecoating. In a preferred embodiment, this proportion is around 70%, forexample between 65 and 75 wt % of the mixture forming the coating.

In the coating, titanium oxide (TiO2) is present in addition to thealumina. A preferred embodiment proposes 70 wt % of alumina (Al2O3) and30 wt % of titanium oxide (TiO2).

According to an alternative embodiment of the invention, all or part ofthe titanium oxide may be replaced by chromium oxide (Cr2O3). Thecoating may thus consist of alumina (50 to 90 wt %), and titanium oxide(0 to 50 wt %) and/or chromium oxide (0 to 50 wt %).

Thus a coating according to the preferred embodiment of the presentinvention contains only “hard” ceramics, and no metal binder betweenthese ceramics is provided.

Comparative tests were performed using a coating according to thepreferred embodiment of the invention and a reference coating of theprior art. The selected coating of the prior art bears the referencePS304. It corresponds to a category described in U.S. Pat. No.5,866,518. Its composition corresponds to a mixture of four components:60 wt % of NiCr as matrix, 20 wt % of Cr2O3 to ensure wear resistance,chemical stability, and high resistance to oxidation, 10 wt % of Ag aslubricant, and 10 wt % of the eutectic BaF2/CaF2 as high-temperaturelubricant.

Resisting torque measurements were made at a temperature of 300° C.While the maximum resisting torque is between 0.30 and 0.35 Nm with acoating of the prior art, it is between 0.25 and 0.30 Nm with a coatingaccording to the invention. As for the residual resisting torque, thisis substantially zero with a coating of the invention while it is about0.01 Nm with a coating of the prior art.

The friction coefficients were also measured. At low temperatures (20°C.), the friction coefficient with a mixture of Al2O3 and TiO2 accordingto the invention is higher (0.19 versus 0.17); however, at highertemperatures the friction coefficient decreases with the invention toabout 0.13 at 300° C., while this coefficient remains almost unchangedat 300° C. (about 0.19) for the PS304 coating.

In terms of wear, the coating according to the invention is ofparticular interest. The wear of the coating was measured in μm3 on ashaft after 10,000 cycles and after 100,000 cycles. With a coating ofthe invention, in both cases the wear is limited to a few (less than 10)μm3, while it is about 150 μm3 at 10,000 cycles, a wear considered toohigh to allow testing at 100,000 cycles.

These various tests show the excellent performance obtained with acoating according to the invention, especially in high temperatureranges. Such performance is unexpected and thus illustrates the interestof the invention.

The invention also relates to a turbomachine part at least partiallycoated with a ceramic self-lubricating coating as described above.

This turbomachine part is, for example, a rotor or at least a circularcylindrical portion of a rotor intended to be placed in a foil-airbearing.

This turbomachine part may also be a thrust disc, in particular a thrustdisc used in combination with a foil-air bearing. It may be arrangedthat at least one face of the thrust disc intended to face the bearingwith which is it associated is coated with a self-lubricating coating.

The turbomachine containing the bearing and a rotor and/or a thrust discaccording to the invention may be any type of turbomachine. Inparticular, it may be a turbomachine driven by an electric motor or aturbomachine driven by a stream of air. The invention also relates tosuch a turbomachine.

The coating described above may be applied to a rotor or to a thrustdisc (or a rotor portion or a thrust disc portion), for example by anytype of thermal spraying process. Preferably, this coating is applied byan air plasma spraying process.

After spraying the coating on the rotor or on the thrust disc (or atleast on a portion of the rotor or disk), the coated surface of the partis ground and/or finished by a vibratory finishing process.

The invention thus enables an air bearing to operate at highertemperatures than bearings of the prior art that use fluoropolymercoatings. A coating according to the invention can be used attemperatures up to at least 400° C., which are the temperaturesencountered at very high rotational speeds and/or in electrically driventurbomachinery.

Indeed, during operation of an air bearing when the rotational speedincreases, the shear of the aerodynamic film also increases. When highspeeds are reached, significant thermal heating therefore occurs whichcauses an increase in the temperature at the bearing (and at the thrustdisc).

In addition, in electrically driven machines, the heat generated by themotor warms the ambient air and also leads to a temperature increase atthe air bearings and thrust discs.

The invention allows working at high temperatures. Tests have alsodemonstrated the high endurance of a bearing with a rotor according tothe invention, as 150,000 startup/shutdown cycles were carried out attemperatures of up to 350° C.

The invention allows effective lubrication of bearings with partsrotating at speeds greater than 60,000 rpm (revolutions per minute) evenin turbomachinery driven by an electric motor.

The invention may find applications for example in devices making use ofturbomachinery, for example air conditioning and/or climate controldevices such as those installed on aircraft for air management in acabin and a cockpit.

Of course, the invention is not limited to the preferred embodiment andto the alternative embodiments presented above by way of non-limitingexamples. It also relates to variants within reach of the skilled personthat lie within the scope of the following claims

1. Use of a coating comprising between 50 and 90 wt % of alumina (Al₂O₃)and an oxide selected from the group consisting of: titanium oxide(TiO₂), chromium oxide (Cr₂O₃), and a mixture of thereof, as aself-lubricating coating for the high-temperature use of a rotor in afoil bearing.
 2. The use of a coating according to claim 1, wherein thecoating consists of a mixture of 50 to 90 wt % of alumina (Al₂O₃) withan oxide selected from the group consisting of: titanium oxide (TiO₂),chromium oxide (Cr₂O₃), and a mixture thereof.
 3. The use of a coatingaccording to claim 2, wherein the coating consists of a mixture ofalumina (Al₂O₃) and titanium oxide (TiO₂).
 4. The use of a coatingaccording to claim 1, wherein the coating comprises between 65 and 75 wt% of alumina (Al₂O₃).
 5. Turbomachine part comprising a circularcylindrical bearing, wherein said bearing is at least partially coatedwith a self-lubricating coating comprising between 50 and 90 wt % ofalumina (Al₂O₃) as well as an oxide selected from the group consistingof: titanium oxide (TiO₂), chromium oxide (Cr₂O₃), and a mixturethereof.
 6. The turbomachine part according to claim 5, wherein saidself-lubricating coating is deposited by an air plasma spraying process,the coated surface then being ground.
 7. The turbomachine part accordingto claim 5, selected from the group of parts consisting of: rotors andthrust discs.
 8. Turbomachine comprising a turbomachine part accordingto claim
 5. 9. The turbomachine according to claim 8, comprising afoil-air bearing having a housing which accommodates, on the one hand, acircular cylindrical portion of a turbomachine part, said circularcylindrical portion being at least partially coated with aself-lubricating coating comprising between 50 and 90 wt % of alumina(Al₂O₃) and an oxide selected from the group consisting of: titaniumoxide (TiO₂), chromium oxide (Cr₂O₃), and a mixture thereof, and on theother hand at least one foil, called the top foil, coming to face saidcircular cylindrical portion, said top foil being untreated.
 10. Airconditioning device, wherein said air conditioning device comprises aturbomachine according to claim 8.